(a) Field of the Invention
The present invention relates to a method for fabricating a surface-emission semiconductor laser device and, more particularly, to a method for fabricating a surface-emission semiconductor laser device which has an improved characteristic of injected current/applied voltage and thus is suited for use in an optical data transmission or optical communication.
(b) Description of a Related Art
Surface-emission semiconductor laser devices, particularly those implemented on GaAs substrates, attract a large attention as light sources for use in optical communication systems in the field of data communication. The surface-emission semiconductor laser device fabricated on a GaAs substrate generally includes a pair of multi-layered semiconductor mirrors, or distributed Bragg reflectors (DBRs), each formed of a plurality of pairs of AlGaAs layers each having a mixed crystal structure, and a GaAs active layer structure sandwiched between the pair of DBRs for emission of laser perpendicular to the main surface of the GaAs substrate.
The surface-emission semiconductor laser device, which emits laser perpendicular to the main surface of a substrate, has an advantage of integration feasibility wherein a large number of laser elements are arranged two-dimensionally on the single substrate. Accordingly, the surface-emission semiconductor laser devices are particularly suitable for applications to parallel optical information processing, such as optical interconnection and optical computing, or a large-scale parallel optical transmission by taking advantage of the parallel optical beams.
The surface-emission semiconductor laser device implemented on an n-type GaAs substrate includes a p-type DBR, which has a larger resistivity compared to the n-type DBR due to the inherent characteristic thereof and a smaller occupied area compared to the n-type DBR due to the device structure. This raises a problem of higher electric resistance of the p-type DBR, which causes a smaller injected current with respect to a voltage applied between the electrodes.
Patent Publication No. 2697455, for example, proposes a surface-emission semiconductor laser device for solving the above problem by incorporating a p-type substrate, which mounts thereon an n-type DBR and an active layer which have a column structure and a p-type DBR formed on the substantially entire surface of the p-type substrate to have a lower electric resistance.
FIG. 1 shows the surface-emission semiconductor laser device described in the above-mentioned publication. The semiconductor laser device 50 includes a p-type GaAs substrate 52, a p-type DBR 54 composed of GaAs/AlAs layers formed on the entire surface of the p-type substrate 52, a column structure formed thereon and including an active layer structure 56, an n-type DBR 58 composed of GaAs/AlAs layers and an n-side electrode 60, and a p-side electrode 62 formed on the p-type DBR 54 separately from the column structure. The laser device 50 emits laser perpendicularly to and through the bottom surface of the substrate 52, as shown in FIG. 1.
In a laser device formed on a p-type substrate, in general, an n-type compound semiconductor layer constitutes the top of the epitaxial layers. In the example of FIG. 1, the top epitaxial layer is the n-type GaAs layer or the AlAs layer implementing the n-type DBR 58.
Since the surface-emission semiconductor laser device generally includes epitaxial layers having complicated structures formed on a small area of the main surface of the substrate, the contact area between the n-type compound semiconductor layer and the n-side electrode is inherently small. This highlights the importance in reduction of the contact resistance between the n-type compound semiconductor layer and the n-side electrode in the surface-emission semiconductor laser device. In a conventional surface-emission semiconductor laser having the p-type substrate, however, the reduction of the contact resistance is not satisfactory, and thus the operational voltage of the surface-emission semiconductor laser device is higher than the desired voltage.